The clinical laboratory has become an increasingly important adjunct to medicine, both in diagnosis and therapy. As the variety of situations in which determinations are desired have expanded, there has been an increasing variety of approaches for measuring the substance of interest. There are many considerations involved in the development of the assay. One consideration is the simplicity of the protocol. The more measurements and steps that are required, the greater the likelihood for error. A second consideration is the concentration range and absolute amount to be measured. A third consideration is the nature of the sample involved. A fourth consideration is the nature of pretreatments which may be required. A fifth consideration is the nature of interfering substances in the samples. A sixth consideration is the intended environment in which the assay is performed and the technical skill of the persons who will perform the assay. This will also involve whether an instrument is to be used or only a visual determination. Thus, each new development provides advantages which find particular applications as to analytes, preparation of reagents, nature of the users, and manner of performance.